Memory storage media, such as magnetic memory disks and magneto-optic memory disks, are typically used in a computer's hard disk drive. In the hard disk drive, a memory disk is mounted on a spindle and rotated by a motor at high speed. A read/write head flies at a low height over the memory disk, e.g., approximately one microinch (.mu."), on an air bearing created by the high speed rotation of the memory disk. Because of the low fly height of the read/write head, it is important to ensure that the surface of the memory disk does not have any defects, such as asperities, that may contact the read/write head during use. Unintentional contact between the read/write head and a defect may damage the read/write head or the memory disk which may result in a hard drive crash.
A memory disk may be tested for defects with a testing mechanism, such as a glide head, an optical scanner, or an acoustic emissions sensor. A glide head, for instance, flies over the surface of the memory disk at a height that is slightly lower than the height at which the read/write head will fly. By lowering the fly height of the glide head, the sensitivity of the glide test may be increased. However, reducing the fly height creates complications because of the reduced distance between the glide head and the surface of the disk. Mounted on the glide head is a transducer, such as a piezoelectric crystal, that produces an electrical output signal in response to motion of the glide head. The output signal of a glide head comprises two components: (1) a noise signal corresponding to the surface roughness of the memory disk, and (2) a signal spike corresponding to the glide head contacting a defect on the surface of the memory disk. The output signal produced by the glide head is transmitted to a glide tester where it is amplified and compared to a fixed threshold level. The glide tester indicates that there is a defect whenever the output signal exceeds the fixed threshold level.
There are other methods of testing memory disks using a fixed threshold level. For instance, an optical scanner generates an output signal by focusing light on the surface of a memory disk and analyzing the reflected or scattered light. The output signal from the optical scanner is then compared to a fixed threshold level in order to determine if any unacceptably large defects exist on the surface of the disk.
Similarly, an acoustic emissions sensor provides an output signal that is compared to a fixed threshold level while testing a memory disk. The acoustic emissions sensor produces an output signal in response to sensing acoustic vibrations from a head flying over the surface of the memory disk. A defect produces a larger amplitude vibration in the head and, consequently, a larger amplitude output signal from the acoustic emissions sensor. The output signal of the acoustic emissions sensor may then be compared to a fixed threshold level.
The industry practice for generating a fixed threshold level is by calibrating the testing sensor prior to testing memory disks. Calibration is performed with a calibration bump disk having at least one bump of known size. Using a calibration bump of the known size allows the output signal of the testing sensor to be normalized. In one calibration method, the threshold level is set using the signal spike generated by the calibration bump. In a second calibration method, instead of adjusting the threshold level, one adjusts the gain used to amplify the output signal until the signal spike generated by the calibration bump is at the target threshold level. Thus, in one method, the gain used to amplify the output signal is held constant, while the threshold level is adjusted to the fixed level; and in the other method, the threshold level is held constant, while the gain used to amplify the output signal is adjusted to a fixed level. In both methods, however, after calibration, the threshold level and the gain of the output signal will remain at their respective fixed levels. The use of either method will be hereinafter referred to as using a "fixed threshold level." Once a fixed threshold level is set, the testing sensor may be used to test memory disks for defects.
As shown in FIG. 1, a calibration bump disk 2 has a bump 4 which has a fixed size and is at a known location on the surface 6 of disk 2. The height of bump 4 on calibration bump disk 2 may vary in accordance with the glide height of the disks to be tested, which is generally about 1.0 .mu." high. Further, the width of bump 4 and the number of bumps may vary depending on the type of bump disk being used. During calibration, a glide head 8 is suspended over bump disk 2 by a suspension arm 10. Of course, if another type of testing sensor was being calibrated with bump disk 2, glide head 8 would be replaced by that sensor. The high speed rotation of bump disk 2, as indicated by arrow 12, generates an air bearing over surface 6 on which glide head 8 flies. A piezoelectric crystal (not shown), mounted on glide head 8, produces an electrical output signal in response to any motion of glide head 8. The output signal is transmitted to a glide tester (not shown), which amplifies the output signal.
When glide head 8 contacts bump 4, the piezoelectric crystal produces an output signal in the form of a signal spike which is transmitted to the glide tester. The amplitude of the signal spike produced by glide head 8 is proportional to the energy transferred to glide head 8 when it contacts bump 4. Using the signal spike caused by bump 4, which has a known size, a fixed threshold level may be set.
FIG. 2 is a graph 20 illustrating the calibration of glide head 8. The angular position of glide head 8 over bump disk 2 is shown on the x-axis 22 and the amplitude of the output signal generated by glide head 8 is shown in millivolts (mV) on the y-axis 24. The output signal 26 comprises a noise signal 28 of approximately 100 mV, and a signal spike 30 of approximately 1000 mV. Noise signal 28 is created by the roughness of the surface of the disk, which affects the air bearing on which glide head 8 is flying. Signal spike 30 represents the contact between glide head 8 and bump 4 at the approximate angular position of 180.degree. on bump disk 2. A fixed threshold level 32 is shown set at the peak of signal spike 30, at approximately 1000 mV. Typically, however, there are small fluctuations in the peak of signal spike 30 and, thus, setting the fixed threshold level at the peak of signal spike 30 is often dependent upon operator subjectivity.
As discussed above, another method of setting fixed threshold level 32 is by adjusting the gain used to amplify output signal 26 until the peak of signal spike 30 is set at the target threshold level.
Once calibrated, the glide tester may then be used to glide test a memory disk using the fixed threshold level. The fixed threshold level and the gain of the output signal remain constant during glide testing. During the glide test, the glide tester amplifies the output signal produced by the glide head and compares the output signal to the fixed threshold level. If the output signal exceeds the fixed threshold level a defect is identified. However, bump disks generally have large diameter bumps, e.g., approximately 50 to 200 .mu.m wide, relative to defects found on the surface of a memory disk, e.g., as narrow as 1.0 .mu.m wide. Because signal spikes are proportional to the energy transferred to the glide head during the head to disk contact, using a fixed threshold level based on a large diameter bump on a calibration bump disk may permit narrow, but high defects to pass during glide testing. In other words, the defect may contact the glide head, but because of the narrow width of the defect, the energy transfer will be minimal creating only a small amplitude signal spike, and the defect may go undetected.
A fixed threshold level requires that each testing sensor, including glide heads, optical scanners, and acoustic emissions sensors, be independently calibrated before being used to test memory disks. This pre-calibration process has associated costs, such as maintenance and service costs, as well as the loss of productivity. Further, calibration using bump disks may result in variations in the magnitude of the fixed threshold levels, because there are inherent variances in the testing mechanisms and in the calibration bump disks. For instance, one glide head may have a more sensitive transducer than another glide head or one bump disk may have bumps with slightly different sizes or shapes than found on other bump disks. There is also operator subjectivity involved in the setting of the fixed threshold level.
Another problem is that memory disks are often intentionally textured or roughened in the contact-start-stop (CSS) zone, while they are as smooth as possible in the data zone. On some memory disks there is a gradual transition in the roughness between these two zones. The fixed threshold level that is appropriate for the data zone may be too sensitive for the CSS zone. Accordingly, as the testing mechanism, such as the glide head, moves from one zone to another, the fixed threshold level must be adjusted at an arbitrary radius of the memory disk. This adjustment results in a test that is too sensitive on one side of the arbitrary radius and too insensitive on the other side. Thus, because there is a gradual change in the surface roughness, accurate testing of the transition area is difficult with a testing mechanism that uses a fixed threshold level.
Another problem that occurs when calibrating a glide head or an acoustic emissions sensor with a bump disk is that the sensors lose sensitivity as they are used after the initial calibration. For instance, as a glide head is used its trailing edge wears out, which reduces the amplitude of the output signals because there is less contact area between the glide head and the defects. This wear similarly affects acoustic emissions sensors. The fixed threshold level set during calibration, however, does not have a corresponding decrease in magnitude. A signal spike that is initially greater than the fixed threshold level may fall below the fixed threshold level after continuous use of the head. Thus, a defective memory disk will be allowed to pass after sufficient sensitivity is lost due to head wear. Consequently, the testing sensor must be frequently recalibrated in order to reset the fixed threshold level. Despite frequent recalibrations, the testing sensor may, nevertheless, lose sensitivity between recalibrations. Further, the frequent recalibrations have associated service and maintenance costs as well as costs associated with a loss of productivity.